Make n-net refuse to start unless using CellsV1

At the Barcenlona summit, we discussed disabling n-net everywhere as a
precursor to removal. The only exception for using n-net is as required
for CellsV1 users, which require it as the only out-of-the-box supported
networking option.

We will remove this check and n-net entirely when those users have a
migration path to neutron and cellsv2.

Depends-On: I47f72eac6a7657bf44319fa5d2df18ce62827f6f

Change-Id: I7309afa5230bcaeeeddb554d95c327d3a614ae6e

nova/cmd/network.py

@@ -25,7 +25,7 @@ from nova.cmd import common as cmd_common
 from nova.conductor import rpcapi as conductor_rpcapi
 import nova.conf
 from nova import config
-from nova.i18n import _LW
+from nova.i18n import _LE, _LW
 from nova import objects
 from nova.objects import base as objects_base
 from nova import service
@@ -39,6 +39,12 @@ LOG = logging.getLogger('nova.network')
 def main():
     config.parse_args(sys.argv)
     logging.setup(CONF, "nova")
+
+    if not CONF.cells.enable:
+        LOG.error(_LE('Nova network is deprecated and not supported '
+                      'except as required for CellsV1 deployments.'))
+        return 1
+
     utils.monkey_patch()
     objects.register_all()
 

nova/tests/unit/cmd/test_cmd_db_blocks.py

@@ -51,6 +51,7 @@ class ComputeMainTest(test.NoDBTestCase):
                               db.api.instance_get, 1, 2)
 
     def test_network_main_blocks_db(self):
+        self.flags(enable=True, group='cells')
         with restore_db():
             self._call_main(network)
             self.assertRaises(exception.DBNotAllowed,

releasenotes/notes/nova-network-only-for-cellsv1-dfb72fb1d3339bb3.yaml

@@ -0,0 +1,6 @@
+---
+deprecations:
+  - Nova network was deprecated in Newton and is no longer
+    supported for regular deployments in Ocata. The network
+    service binary will now refuse to start, except in the special
+    case of CellsV1 where it is still required to function.